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The AppModelBuilder does not rely on content so much, rather
it's highly related to the extension system. Therefore, it's
better to be in athena/extensions, and its name should be
ExtensionAppModelBuilder instead.

This will simplifies DEPS rules of athena/content.

BUG=None
R=oshima@chromium.org
TBR=yoz@chromium.org, xiyuan@chromium.org, tony@chromium.org
TEST=build

Review URL: https://codereview.chromium.org/543263002

Cr-Commit-Position: refs/heads/master@{#294011}